feat(medusa): Extract cron job logic to its own service (#2821)

**What**
Extract cron job logic from the `EventBusService` to its own service `JobSchedulerService`

**Why**
Preliminary step to extracting the event bus into a module

**Tests**
Tested with a local installation of Medusa

Resolved CORE-918

www/docs/sidebars.js

@@ -335,8 +335,8 @@ module.exports = {
         },
         {
           type: "doc",
-          id: "advanced/backend/cron-jobs/create",
-          label: "Create a Cron Job"
+          id: "advanced/backend/scheduled-jobs/create",
+          label: "Create a Scheduled Job"
         },
         {
           type: "doc",